A number of different atmospheric pressure ionization (API) sources have been developed for producing ions from a sample at atmospheric pressure. One well-known and important example is the electrospray ionization (ESI) source. The electrospray ionization technique, and more specifically electrospray ionization sources interfaced to mass spectrometers, has opened a new era of study for the molecular weight determination of labile and involatile biological compounds. In electrospray ionization, singly or multiply charged ions in the gas phase are produced from a solution at atmospheric pressure. The mass-to-charge (m/z) ratio of the ions that are produced by electrospray ionization depends on the molecular weight of the analyte and the solution chemistry conditions. Fenn et al. in U.S. Pat. No. 5,130,538 describes extensively the production of singly and multiply charged ions by electrospray ionization at atmospheric pressure.
Briefly, the electrospray process consists of flowing a sample liquid through a small tube or needle, which is maintained at a high voltage relative to a nearby surface. The voltage gradient at the tip of the needle causes the liquid to be dispersed into fine electrically charged droplets. Under appropriate conditions the electrospray resembles a symmetrical cone consisting of a very fine mist of droplets of ca. 1 μm in diameter. Excellent sensitivity and ion current stability is obtained if a fine mist is produced. Unfortunately, the electrospray “quality” is highly dependent on the bulk properties of the solution that is being analyzed, such as for instance surface tension and conductivity. The ionization mechanism involves desorption at atmospheric pressure of ions from the fine electrically charged particles. In many cases a heated gas is flowed so as to enhance desolvation of the electrosprayed droplets. The ions created by the electrospray process are then mass analyzed using a mass analyzer.
In U.S. Pat. No. 5,171,990 there is described an electrospray ion source of the type which includes an ion transfer tube communicating between the ionizing region and a low-pressure region with a skimmer having an aperture through which ions pass. The skimmer separates the low-pressure region from a progressively lower pressure region, which includes ion focusing lenses and an analyzer. The ion transfer tube is oriented so that undesolvated droplets or particles traveling through the tube are prevented from passing through the skimmer aperture into the analysis region. In particular, the axis of the ion transfer tube is altered or directed so that the axis is offset from the skimmer aperture. In this way, there is no alignment between the bore of the tube and the skimmer aperture. The tendency is for the large droplets or particles to move to the center of the flow in the ion transfer tube and travel in a straight line. These droplets or particles traveling in a straight line strike the skimmer. The droplets or particles are thereafter pumped away. Additionally, a tube lens is provided adjacent to the outlet end of the ion transfer tube for focusing and/or diverting ions toward the skimmer aperture. Unfortunately, since the ions follow an off-axis trajectory through the skimmer aperture there is a tendency for some of the ions to continue along a trajectory terminating at a surface of an ion transfer element adjacent the exit side of the skimmer. Over time, a burn/deposit becomes apparent on the surface of the ion transfer element that is opposite the ion transfer tube. This effect reduces the throughput of the ion source, and thereby reduces the overall sensitivity of the instrument.
Accordingly, there is a need for a system that increases the throughput of the ion source while at the same time maintaining low chemical background noise.